Electronic Nematic Phases in Correlated Electron Systems

When water is cooled it forms ice. The properties of ice and water are dramatically different. The change of state from water to ice is a common example of a "phase transition". When ice forms, the water molecules order and the symmetry of the system is lowered. In this project, we will investigate the properties of a number of systems which have "electronic nematic phases" (ENP's). The materials to be investigated are layered ruthenates and iron-based superconductors. The common feature of the these materials is that the electronic properties (such as the electrical resistance) become anisotropic at low temperatures without a change of crystal symmetry. This suggests that the current carrying electrons (conduction electrons) form a new "ordered" state or electronic nematic phase at low temperature.

The importance of electronic nematic phases in condensed mater has been recognized in the last few years. They occur in two dimensional semiconductor materials and the ruthenate material Sr3Ru2O7 at high magnetic field, high-temperature (cuprate) superconductors, and the recently discovered iron-based superconductors. The objective of the present project is to relate the anisotropic electronic behaviour of two types of ENP (iron-based superconductors and ruthenates) to their incipient magnetic properties. We will use neutron and x-ray scattering to investigate the collective magnetic excitations in these materials. Experiments will be performed at international facilities such as the ISIS spallation source in the UK and ILL in France. Our preliminary results suggest that the collective magnetic excitations are key to understanding the ENP behavior in both these systems. New instrumental advances will enable us to create and study the ENP phases in situ for the first time by applying a uniaxial stress or high magnetic field.

The proposed research will result in advances in our scientific and technical knowledge. These will benefit other researchers and academia as whole in a number of ways. We will be working closely with other researchers in the project who will benefit directly. For example, interactions with our collaborators in Edinburgh and Stanford will allow the relationship between the spin correlations and the bulk physical properties of electronic nematic systems to be understood. The interaction will lead to new avenues of research being opened up. We will also work with colleagues around the world to interpret our results and relate them to the physical properties of electronic nematic phases. This will be achieved by giving presentations at other institutions, and attending workshops and conferences. There are many models purporting to describe electronic nematic phases and our results will allow these to be verified or disproved. We will work with our theoretical colleagues to achieve this goal by providing input for theoretical models. In order to active maximum impact we will aim to publish some of the work in the highest profile journals.

This proposal is to fund fundamental research into strongly correlated electronic systems (SCES). SCES's already have practical applications and offer promise for the future. For example, they are used in power transmission (superconductors), data storage (MRAM and magnetoresitive materials) and motors (rare earth magnets). The discovery of new classes of SCES such as the high-temperature cuprate superconductors and, more recently, the iron-based superconductors has ignited the field. Interesting physical behaviour such as superconductivity is usually related to a change in the underlying symmetry or order of the system. The long term goal of basic research in strongly correlated electronic systems, such as that proposed here, is to understand the mechanisms which can cause novel and potentially useful physical behavior. This will lead to better materials. One area of particular interest is superconductivity. Superconductors already have industrial applications. The biggest application at the moment is to produce the magnetic fields required for magnetic resonance imaging (MRI). This is a multi-billion dollar industry in which the UK has a significant position through companies such as Siemens Magnet Technology, Oxford Instruments and Cryogenic Ltd. Superconducting magnets are also currently used in maglev trains, for providing confinement fields for fusion reactors (eg. tokamaks) and beam steering in particle accelerators. Siemens has recently produced a compact efficient 4MW motor which uses superconducting coils. The use of high-temperature superconductors in industrial applications will potentially open many new avenues. The main applications foreseen include: power transmission; power generation; electric motors; and fault current limiters. A recent example of a pilot power transmission scheme is the high temperature superconducting underground cable installed for the Long Island Power Authority (LIPA) this carries 574 MW of power.

The research project will provide training and development for the post-doc and student involved in the work. Experimental skills will be acquired in the areas of crystal growth and characterization, low temperature experimental techniques, neutron scattering and data analysis. The project will also enable us to engage undergraduate students at Bristol who would perform final year projects in this area of research.